The term “plasma” describes a gaseous particle system containing balanced charges of free ions and electrons. In contrast to the quasi-neutrality of plasma, the charge carriers cause various physical phenomena, such as the electric conductivity of plasma, the appearance of oscillations, and the formation of boundary sheaths if in contact with solid surfaces. Depending on its temperature, plasma can be described as either low-temperature plasma (LTP), or high-temperature plasma (HTP). Plasma generated at ambient conditions, i.e. about 300K and at atmospheric pressure, is denoted as non-thermal plasma (NTP).
Devices generating a free NTP jet, so-called “cold plasma jets” or “NTP probes,” allow the localized application of plasma to objects. Several technical designs for the generation of cold plasma beams have been reported. Many such devices are based on the principle of a dielectric-barrier discharge (DBD) and are characterized by the coverage of at least one electrode with dielectric material. High voltages and high frequencies drive the generation of plasma. Typically, the geometry, i.e., the length and diameter of the plasma jet, is determined by the geometry of the NTP device.
NTP jets enable numerous technical applications. Recently, the use of NTP-based devices was introduced for the ionization of molecules and their subsequent analysis using mass spectrometry. This strategy does have various advantages compared to conventional ionization methods, most remarkably the operation at ambient conditions and the possibility of screening large samples.
According to a review on the state of the art, current proposals in the field of interest are as follows:
The invention called “Inductively coupled plasma mass spectrometer” for the U.S. Pat. No. 6,265,717 granted on Jul. 24, 2001, where Kenichi Sakata et. al., disclosed an apparatus and method for inductively coupled plasma mass spectrometry (ICP-MS) with improved detection limits. The ICP-MS includes the devices for generating inductively coupled plasma (ICP) in a gas at substantially atmospheric pressure to ionize a sample, a mass analyzer (MS) operable at a low pressure of 10−2 to 10−4 Pa for detecting at least part of the sample ions, and an interface for transferring the sample ions from the ICP to the MS. The interface is provided with a regulator to increase the interface pressure of its normal pressure; for example, from 350 to 450 Pa. The increased pressure may reduce the sensitivity of the instrument, but can improve the detection limits by reducing the ion-selective interference.
The U.S. Pat. No. 6,248,998 granted on Jun. 19, 2001 to Toyoharu Okumoto, et al., entitled “Plasma ion source mass spectrometer” discloses an ion mass spectrometer comprising a plasma ion source for ionizing a sample with plasma; a mass filter for subjecting the sample ionized by the plasma ion source to mass spectrometry; and an interface unit having an orifice formed in a cone for introducing the ionized sample ionized by the plasma ion source into the mass filter. Further, the plasma ion source mass spectrometer comprises a first cooler for cooling a plasma generator of the plasma ion source and a plasma-generating power source; and a second independent cooler for cooling the interface unit and for raising the temperature by changing the cooling efficiency to reduce the influence of deposition on the interface unit. With this construction, the temperature of the interface unit can be controlled without changing the analysis sensitivity of the plasma ion source mass spectrometer.
By the other hand, U.S. Pat. No. 5,616,918 granted on Apr. 1, 1997 to Konosuke, Oishi, et. al., discloses the invention “Plasma ion mass spectrometer and plasma mass spectrometry using the same,” referring to a plasma ion mass spectrometer capable of improving detection accuracy in mass spectrometry by reducing background noise owed to ultraviolet radiation and neutral particles and ion mass spectrometry using the same plasma. A sample is ionized with a plasma-generating portion. The ionized sample flow is shielded by a shielding means after an elapse of a specified time, and the ions of the sample accumulated before shielding the flow of the ionized sample are held for a specified time. Then, mass spectrometry is performed for ions of the sample held for the specified time. While the sample ions accumulate before shielding, ultraviolet radiation mixed with the ions of the sample disappears. Hence, only the ions of the sample can be held. So the background noise is reduced, and the detection accuracy in mass spectrometry is improved.
Finally, U.S. Pat. No. 5,223,711 granted to Neil Sanderson and Christopher Tye on Jun. 29, 1993 discloses the invention “Plasma sources mass spectrometry,” referring to an improved apparatus and method for plasma source mass spectrometry, the apparatus comprising means for generating plasma at substantially atmospheric pressure in a gas; means for introducing a sample to the plasma wherein the sample is ionized to form sample ions; means for transmitting the ions from the plasma into an evacuated chamber; a mass filter disposed within the evacuated chamber; a substantially non-multiplying ion detector comprising an ion collector, the detector being responsive to the charge of at least some of the sample ions which pass through the mass filter; and means for inhibiting the response of the detector to electrically neutral particles. Typically, the detector comprises a suppressor and means for negatively biasing the suppressor with respect for the collector, and a shield disposed to shield the suppressor from the neutral particles. Improvements include a greater dynamic range with reduced sensitivity to noise.
Biomedicine is another area of potential importance for NTP jets. Several reports describe the efficient inactivation of microbes and spores. This could be harnessed, for example, to sterilize root canals in teeth. Human cells could undergo low-temperature plasma treatment in vitro without causing any detectable damage. However, still more information is required to assess the safety of the application of plasma. Particularly, there is a lack of in vivo studies evaluating the effect of NTP on intact higher organisms.
In the present patent application entitled “Non-thermal plasma jet as source of spatial ionization source for ambient mass spectrometry and method for its application”, we present a novel device comprising an NTP jet based on a double discharge dielectric barrier from which the NTP jet coupled to a mass spectrometer can detect chemical compounds directly from an intact organism without causing significant damage.